How to Make It
Preheat broiler with rack 6 inches from heat. Toss scallions with olive oil and ¼ teaspoon each salt and pepper on a rimmed baking sheet. Broil until beginning to brown, 5 to 7 minutes.
Meanwhile, season fish with ¼ teaspoon each salt and pepper. Whisk mustard, sesame oil, ginger, and remaining ¼ teaspoon each salt and pepper in a small bowl; spread over tops and sides of fish.
Nestle fish among scallions and broil until fish is just cooked through and scallions are charred in spots, 6 to 10 minutes. Serve with steamed rice, if desired.